A parallel-operating hybrid drive train with serial arrangement of components in the aforementioned manner is possible in different embodiments.
Known from DE 103 46 640 A1, for example, is one such hybrid drive train in which the electric motor is arranged coaxially about the input shaft of the driving transmission, the rotor of the electric motor is attached directly to the input shaft of the driving transmission in a non-rotational manner, and the driving transmission is designed as an automatic planetary transmission.
Described in DE 100 12 221 A1 is a hybrid drive train with a primary drive train and a secondary drive train. In the primary drive train which corresponds to the hybrid drive train under consideration herein, the relevant electric motor is arranged axis-parallel to the input shaft of the drive transmission and the rotor of the electric motor is in a drive-connection with the input shaft of the drive transmission by means of an input transmission stage with high translation ratio (iEK>1) designed as a pair of spur gears. The drive transmission is designed preferably as an automatic planetary transmission. The discussion below relating to the torque and the rotational speed of the electric motor transmission in this kind of arrangement of the electric motor applies accordingly to the reduced values applied to the output element of the input transmission stage and/or to the input shaft of the drive transmission.
In the above referenced invention the drive transmission is designed preferably as an automatic load-shift transmission, such as an automatic planetary transmission, a double clutch transmission or a stepless transmission. The drive transmission, however, can also be designed as an automatic shift transmission with countershaft design in which shifting processes are associated with an interruption in traction power.
In addition, a start-up element can be provided directly in front of the drive transmission. For example, a hydraulic torque converter can be connected upstream to an automatic transmission in a known manner which is bridged in standard drive mode, that is, outside of start-up and backing processes, by means of an engaged lock-up clutch. A start-up clutch or start-up and shifting clutch designed as dry clutch, in particular as membrane spring clutch or as wet clutch, in particular as laminar clutch which is normally engaged, can be connected upstream to a stepless transmission and to an automatic shifting transmission. The same also applies to a double clutch transmission, which is known to consist of two input shafts each with an associated start up and shifting clutch. Alternatively, a start-up element can also be integrated into the drive transmission, for example, by placement of a start-up clutch associated with the input shaft within the transmission housing of the drive transmission or in that a load-bearing, friction shift element of a drive transmission, designed as automatic transmission, is constructed as a start-up clutch.
This kind of hybrid drive train offers the possibility of operating a motor vehicle, if necessary, in a purely internal combustion drive mode, in a purely electric drive mode, or in a combination drive mode. In the internal combustion drive mode the clutch is engaged, the electric motor is disconnected and the motor vehicle is driven in traction mode solely by means of the drive torque of the internal combustion engine, and in the motor-braking mode—potentially in addition to the braking torque—the motor vehicle is decelerated by additional braking devices, such as an operating brake or a retarder, supported by the drag torque of the internal combustion engine then running in motor-braking mode. In the electric drive mode, the clutch is disengaged, the internal combustion engine is switched off and the motor vehicle is being driven in traction mode solely by the torque of the electric motor operating then as an engine, and in motor-braking mode—perhaps additionally supported by other brake features in addition to the braking torque—the vehicle is decelerated by the drag torque of the electric motor then operating as generator.
In combination drive mode, the clutch is disengaged and the motor vehicle is driven in traction mode by the sum of the drive torques of the internal combustion engine and of the electric motor, and in motor-braking mode—perhaps additionally supported by other brake features in addition to the braking torque—the vehicle is decelerated by the sum of the drag torque of the internal combustion engine and by the electric motor then operating as generator.
In addition to the hybrid drive modes under consideration here, the internal combustion engine and the electric motor can also be operated, if necessary, with a different direction of power flow, so that the generated torques are partly cancelled out. For example, in certain operating phases of traction mode, it may be useful to operate the electric motor as a generator, opposite to the effect of the drive torque of the internal combustion engine, for instance, in order to recharge a drained electric power supply or to operate the internal combustion engine at an optimum operating point. Likewise, in certain operating phases of motor-braking it may be useful to operate the electric motor as an engine, opposite to the effect of the drag torque of the internal combustion engine, for instance, to keep the internal combustion engine above a critical rotational speed limit.
Based on the large number of different operating characteristics of the potential embodiments of this kind of hybrid drive train, known control methods usually are postulated on at least one determination of the power of the electric motor and/or on a particular design of the drive transmission. In addition, the known control methods are often restricted to the solving of partial problems occurring in the control of the particular hybrid drive train.
A method to control a corresponding hybrid drive train is disclosed in DE 43 24 010 C2 which proceeds from a design of the drive transmission as an automatic planetary transmission with a hydraulic torque converter connected immediately upstream. The known method provides that the electric motor be controlled in pure electric drive mode, such that the torque characteristic of an internal combustion engine is simulated. Furthermore, the known method provides that in the motor-braking operation of the relevant motor vehicle the braking moment (drag torque) of the internal combustion engine is supplemented by, or replaced, by a braking torque of the electric motor produced in generator operation.
A similar method to control a corresponding hybrid drive train is described in DE 101 50 990 A1. This known method also proceeds from the design of the drive transmission as an automatic planetary transmission, but no hydraulic torque converter is connected upstream thereto. This method also provides that the electric motor is controlled in a pure electric drive mode, such that the operating behavior of the internal combustion engine is simulated. In a change from electric drive mode (with idling internal combustion engine and disengaged clutch) into combination drive mode or into internal combustion drive mode, the internal combustion engine is started by engaging the clutch, whereby a soft, that is low-jolt, transition to internal combustion engine power is to be ensured. DE 101 50 990 A1 does not, however, indicate how the clutch and the electric motor are actually to be controlled, in order to achieve this result.
An additional method to control a corresponding hybrid drive train is known from DE 102 60 435 A1 which proceeds from an embodiment of the drive transmission as an automatic shift transmission, a double clutch transmission or a stepless transmission, having a second clutch (start-up and shifting clutch or start-up clutch) connected immediately upstream. This method provides that in a change from electric drive mode (with internal combustion engine shut off, disengaged first clutch and engaged second clutch) into combination drive mode, the internal combustion engine is started by engagement of the first clutch, whereby during the starting of the internal combustion engine the power output from the electric motor is increased to avoid a drop of torque on the output side, and the second clutch is partly disengaged to avoid torque fluctuations on the output side or is operated at the slippage limit. No additional data, however, is provided in DE 102 60 435 A1 about the actual control of the first clutch and of the electric motor during starting of the internal combustion engine.
Starting of the internal combustion engine by engaging the clutch located between engine and the electric motor, however, is problematic, since the breakaway torque for cranking of the internal combustion engine and the drag torque (which must be subsequently overcome to accelerate the internal combustion engine up to the rotational speed which allows start-up of the internal combustion engine) are essentially dependent on operating parameters of the internal combustion engine, such as the engine temperature (coolant temperature and oil temperature) and on the maintenance and wear state of the internal combustion engine.
Thus the breakaway torque and the starting drag torque in old internal combustion engines and/or in internal combustion engines in a poor state of maintenance and repair are much higher than for warmed-up internal combustion engines and/or for internal combustion engines in a good state of maintenance and repair. If the start-up process of the internal combustion engine, in particular the wear on the clutch, is performed without taking account of the relevant operating parameters, there necessarily results different starting times for starting of the internal combustion engine and accordingly different control processes for changing from the electric drive mode into the combination drive mode or into the internal combustion drive mode.